The present invention relates to an auxiliary device, a marine surface vessel, and a method for corrosion protection in a marine construction, such as a marine surface vessel or a marine structure, the marine construction comprising a plurality of metal elements and at least one reference electrode, the metal elements and the reference electrode being at least partly immerged in an electrolyte in the form of water, in which the marine construction is at least partially immerged, the metal elements comprising an anode and a metal part, the anode being provided for corrosion protection of the metal part by the provision of an electrical protection current through an electrical circuit comprising the anode, the metal part and the electrolyte, the method comprising measuring an electric potential of the metal part with the reference electrode as a ground reference.
In marine constructions, such as marine vessels and marine structures; a known way to protect an immersed metal part against galvanic corrosion is to provide a sacrificial anode, made of very pure zinc, magnesium, cast iron or an alloy of aluminum, which is directly fastened to, or electrically connected via a cable to the immersed metal part. In such a system, herein referred to as a passive corrosion protection system, or simply a passive system, the sacrificial anode will waste away, preventing damage to the immersed metal part. An alternative to passive arrangements with sacrificial anodes is impressed current cathodic protection (ICCP) systems. In such a system, one or more active anodes are immersed and connected along with the immersed metal part to a common DC electrical power source. ICCP system active anodes usually have tubular or solid rod shapes, or are provided as continuous ribbons, and can include materials such as titanium, platinum, high silicon cast iron, graphite, mixed metal oxide, and niobium. Regardless whether the corrosion protection system is a passive system or of the ICCP type, a circuit is provided by an electrical current through the water serving as an electrolyte, and a surface polarization at the interface between the metal part and the water is created, serving to protect the metal part against corrosion.
In a passive system, as well as in an ICCP system, for a good corrosion protection, the electrical current between the anode and the metal part should be such that the surface polarization of the metal part is kept close to a desired value, or a desired interval, which depends on the material of the metal part. If said current is too low, the protection is too low, and the metal part may corrode at an undesired rate. For some metals, e.g. aluminum, damages can occur also if the current is too high. In general, too high of a current between the anode and the metal part gives calcium oxide precipitation which can stimulate excessive growth of sea weeds and sea animals on the metal part. Such growth is a particularly large concern in the case of recreational boats, which often stay docked giving opportunities for the growth.
ICCP systems usually include an electronic control unit (ECU) by means of which the electrical current between the anode and the metal part can be varied and controlled, which allows for control of the current between the anode and the metal part through the electrolyte, and thereby control of the surface polarization at an interface between the metal part and the electrolyte. Also passive systems can include such control, which can be obtained by an adjustable resistance in the electrical connection between the sacrificial anode and the metal part, for example as described in U.S. Pat. No. 5,627,414.
The control of the current is carried out based on measurements of a parameter indicative of said polarization. Such measurements are obtained by means of a reference electrode also immersed in the electrolyte. More specifically, the ECU is adapted to measure an electrical potential of the metal part with the reference electrode as a ground reference, and to control the current between the anode and the metal part based on these measurements and the desired value, or desired interval, of the electrical potential of the metal part with the reference electrode as a ground reference. This desired value, or desired interval, depends on a number of circumstances, for example, the type of reference electrode used, and the type of electrolyte. As an example, where the metal part is made of a material from a certain group of copper alloys, and the electrolyte is salt water, the desired value of the electrical potential of the metal part with a silver chloride coated silver reference electrode as a ground reference is suitably within the interval −450 mV-(−600) mV. As a further example, where the same type of reference electrode is used in the same type of electrolyte, and the metal part is made of a material from a certain group of aluminum alloys, the desired value of the electrical potential of the metal part with the reference electrode as a ground reference is suitably −950 mV.
A problem with silver chloride coated silver reference electrodes, below also referred to as silver/silver chloride reference electrodes, is that the silver chloride coating thereof can be partially or completely lost for example by physical wear, and/or by polarization due to an external electrical field. This problem is also present at reference electrodes with an electrically conductive core, for example a copper core, a silver layer outside the core and a silver chloride layer outside the silver layer, below referred to as core/silver/silver chloride reference electrodes. A possible reason for reference electrode damage on recreational boats is the use, while the boat is lifted, of high pressure washers, the jet of which can remove the silver chloride coating of exposed reference electrodes. The electrical potential measurement provided with a reference electrode on which the silver chloride coating is partially or completely lost will be inaccurate, and the measured value will differ from the value that would have been obtained with an intact reference electrode. In turn this will provide an erroneous input for the control of the electrical current between the anode and the metal part, resulting in a less than optimal corrosion protection, and possibly even damage to the metal part.
For boats, a known measure for taking care of damaged reference electrodes is the obvious one of lifting the boat, and replacing the electrodes, which of course involves the need for expensive replacement electrodes, and adds to the maintenance of the boat.
It is desirable to improve the corrosion protection in marine constructions.
It is also desirable to reduce errors in the measurement of the electrical potential of a metal part with a reference electrode as a ground reference in a marine construction.
It is a further desirable to provide an accurate corrosion protection in marine constructions, while keeping maintenance of the marine construction low.
According to an aspect of the present invention, a method comprises connecting at least one of the metal elements and at least one of the at least one reference electrode to a DC electrical power source so as to allow an electrical regeneration current through an electrical circuit comprising the at least one of the metal elements, the at least one of the at least one reference electrode and the electrolyte, so as for the reference electrode to be anodized.
By the method according to an aspect of the invention, a regeneration of the reference electrode will provided by it being anodized, which in the case of silver/silver chloride reference electrodes or core/silver/silver chloride reference electrodes will reestablish damaged or worn off silver chloride layers. This in turn will eliminate the risk of erroneous inputs for the corrosion protection, securing an accurate protection. Also, such reference electrode regeneration can be carried out at any time while the marine construction is in its normal immersed state, without the need for time consuming and expensive maintenance, such as lifting a boat and replacing the electrodes.
It should be noted that for proper anodizing of the reference electrode, it should be connected to a positive pole of the DC electrical power source and the metal element should be connected to a negative pole of the DC electrical power source. It should also be noted that the metal element forming a part of the electrical circuit for anodizing the reference electrode does not necessarily have to be identical with the metal part that is to be protected from corrosion. For example, the at least one of the metal elements, connected along with the at least one of the at least one reference electrode to the DC electrical power source so as to allow the electrical regeneration current, can be the anode provided for corrosion protection of the metal part. Thereby, the anode can be an active anode provided for impressed current cathodic protection of the metal part, or a sacrificial anode in a passive system. However, in a preferred embodiment, the at least one of the metal elements, connected along with the at least one of the at least one reference electrode to the DC electrical power source so as to allow the electrical regeneration current, is the metal part for which the anode is provided for corrosion protection.
It should further be noted that the invention is applicable to any corrosion protection system where reference electrodes are used to measure the surface polarization of the metal part to be protected, regardless whether the system is active or passive. In addition, it should be noted that since the invention concerns marine constructions, the electrolyte is sea water, i.e. salt water, brackish water or freshwater. As exemplified below, the DC electrical power outlet can be fed from a separate DC electrical power source, or it can be identical with a DC electrical power source.
Preferably, the method comprises measuring separately the electric potential of one of the metal elements with a respective of at least two reference electrodes as a ground reference, and connecting at least one of the metal elements and at least one of the reference electrodes to the DC electrical power source, if the electrical potential measured with one of the reference electrodes differs substantially from the electrical potential measured with another of the reference electrodes.
This given a secure manner to establish whether there is a need for regeneration of any of the reference electrodes. For example, where the corrosion protection system comprises two reference electrodes, and the electrical potential of the metal part is measured with each of the two reference electrodes as a ground reference, if the electrical potential measured with one of the reference electrodes differs substantially from the electrical potential measured with the other reference electrode, this is an indication that at least one of the reference electrodes is defective. Thereby, both reference electrodes can be connected to the positive pole of the DC electrical power source, so as for them to be anodized.
It should be noted that, notwithstanding that the reference electrodes are used to measure the electric potential of the metal part provided with corrosion protection, the step mentioned above of measuring the electric potential can also be seen as a calibration measure for the reference electrodes, and as such it can involve measuring the electric potential of the metal part provided with corrosion protection, or any other metal element of the marine construction. For example, in a passive system, a metal element used for such a calibration measure can be the sacrificial anode. However, in an ICCP system, the active anode is often not suitable to be used as a metal element for a calibration measure of said type since it is usually electrically charged.
Preferably, the step of connecting at least one of the metal elements and at least one of the reference electrodes to the DC electrical power source is carried out if the electrical potential measured with one of the reference electrodes differs from the electrical potential measured with another of the reference electrodes by at least a predetermined threshold value. Where two reference electrodes are used the difference electrical potential measurements is easy to establish. In systems with three reference electrodes, the measurement difference compared to the predetermined threshold value can either be the difference between the highest and the lowest measured values, or the largest difference between two measured values without any measured value between them. The latter alternative provides a narrower limit for the divergence between the measured values.
Preferably, the predetermined threshold value is at least 5 mV. Thereby, preferably the measurement difference compared to the predetermined threshold value is the largest difference between two measured values without any measured value between them. Specifically, the predetermined threshold value can be in the interval 5-100 mV. In more preferred embodiments, the predetermined threshold value can be in the interval 5-75 mV, advantageously in the interval 20-50 mV.
Preferably, the method comprises measuring separately the electric potential of one of the metal elements with a respective of three reference electrodes as a ground reference, and connecting, if the electrical potential measured with one of the reference electrodes differs substantially from the electrical potentials measured with the two other reference electrodes, at least one of the metal elements and the reference electrode, with which the substantially differing electric potential was measured, to the DC electrical power source.
An advantage with using three reference electrodes in this manner is that if only one of the reference electrodes is defective and needs to be regenerated, this reference electrode can be identified. This means that only the reference electrode identified as defective needs to be subjected to the step mentioned above so as to be anodized. As an example, the method can comprise measuring the electrical potential of the metal part with a first, a second and a third reference electrode as ground references, and connecting, if the electrical potential measured with the first reference electrode differs substantially from the electrical potentials measured with the second and third reference electrodes, the first reference electrode to the positive pole of the DC electrical power source, so as for it to be anodized.
Also, in certain applications, carrying out on one of the three reference electrodes the regeneration might have to be followed by a time period during which a remaining electric potential of the reference electrode, caused by the regeneration, decreases, before the reference electrode is used for measurements in the corrosion protection system. During this time period the other two reference electrodes may be used for measurements in the system.
It should be noted that where a plurality of reference electrodes are used as stated above, they should be galvanically separated from each other.
Alternatively, the method can comprise performing repetitively, at predetermined time intervals, the step of connecting the at least one of the metal elements and the at least one of the at least one reference electrode to the DC electrical power source. Thereby, the corrosion protection can be carried out with only one reference electrode. In addition, instead of being used for total renovation of a malfunctioning silver/silver chloride reference electrode or core/silver/silver chloride reference electrode, the method can be used for maintaining such a reference electrode, by carrying out the regeneration relatively often. Thereby, the regeneration can be carried out during a relatively short time, for example once a week for 30 seconds, or once a month for one minute. Below, some advantageous values for the current density during the regeneration are given.
Preferably, the method comprises controlling the electrical regeneration current through the electrical circuit comprising the at least one of the metal elements, the at least one of the at least one reference electrode and the electrolyte. Since a correct level of the regeneration current is important for the anodizing process of the reference electrode, by such current control it is possible to secure a successful result of the regeneration. Preferably, said electrical regeneration current is controlled by control of a variable resistance in said electrical circuit.
Preferably, the electrical regeneration current is such that the current density at the reference electrode is 0.1-250 mA/cm2, in more preferred embodiments 0.1-100 mA/cm2, in yet even more preferred embodiments 1-40 mA/cm2. In particularly preferred embodiments said current density is 3-18 mA/cm2. In this current density interval the marine construction corrosion protection reference electrode will be regenerated at current density levels provided by tests presented in “Reference electrodes: Theory and practice”, David J G Ives & George J Janz, Academic Press, New York 1961. In mostly preferred embodiments said current density is 5-12 mA/cm2.
The current density at the reference electrode is the electrical regeneration current divided by the surface area of the reference electrode. It should be noted that said current density values are suitable for a silver/silver chloride reference electrode, or a core/silver/silver chloride reference electrode. The current density can be controlled for example as mentioned above, i.e. by controlling the electrical regeneration current by control of a variable resistance in said electrical circuit. Alternatively, the current density can be controlled by controlling the electrical tension (voltage) between the metal element and the reference electrode.
In embodiments mentioned above, where more than one reference electrode is provided, and the electrical potential measured with one of the reference electrodes differs substantially from the electrical potential measured with another of the reference electrodes, the regeneration might be carried out in order to provide a total renovation of one or more of the electrodes. Such a total renovation might of course also be carried out where only one reference electrode is provided. Suitably, if a total renovation is desired, the regeneration is carried out at said current density values, preferably at 3-18 mA/cm2, during 2-30 minutes. A test performed by the inventor suggested that a regeneration at a current density of 5-12 mA/cm2, preferably approximately 8 mA/cm2 during approximately 15 minutes provides a very good result. In general, if a relatively high current density is provided, the duration of the regeneration can be relatively short, and vice versa. However, the use of current densities that are too high, or regeneration durations that are too long, might provide a silver chloride layer that easily releases from the silver core, (or the silver layer provided on the electrically conductive core).